1. Field of the Invention
The present invention relates to a voltage/current conversion circuit suitable for applications in integrated circuits.
2. Description of the Related Art
An example of the conventional voltage/current (v/c) conversion circuits used generally for converting input voltage to output current is shown in FIG. 5. In this v/c conversion circuit shown in FIG. 5, if the gain of the operational amplifier (OP amp) is ideally high, a negative feed back is generated between the OP amp 2 and an n-type metal oxide semiconductor field-effect transistor (MOSFET) 5 serving as the active load so that the voltage input terminal 1a and the node point 1b are at the same potential. Denoting the potential at the voltage input terminal 1a as Vin volts, the resistance of the resistor element 46 as R ohms, and the potential at the node 1b as Vin volts, the current Ir flowing in the resistor element 46 is given by Vin/R in amperes.
In FIG. 5, a p-type MOSFET 3 and another p-type MOSFET 4 constitute a current mirror so that the magnitude of the current flowing in p-type MOSFET 4 is the same as that flowing in p-type MOSFET 3. Therefore, the magnitude of the current flowing out of the output terminal 8, Iout, becomes the same as that flowing in the resistor element 46 given by Vin/R amperes.
The v/c conversion circuit shown in FIG. 5 is characterized by a good linearity of the input/output relation, because the voltage/current conversion ratio between the input potential Vin and the output current Iout is determined by the value of the resistor R. However, when it is desired to provide a v/c conversion circuit that produces a small variation in the output current for a given variation in the input potential, i.e., to make a voltage/current conversion circuit with a low value of voltage/current conversion ratio, it is necessary to have a high resistor R. In IC production processes, high resistor elements are not desirable, because they require an increase in the substrate area or an increase in the number of fabrication steps.
Also, the circuit shown in FIG. 5 does not permit a selection of the level of the output current, independently of the voltage/current conversion ratio. If the resistor value R is lowered to increase the output current level, the voltage/current conversion ratio inevitably becomes high (refer to FIG. 6, line b), and conversely, if the resistor value R is increased to decrease the voltage/current conversion ratio, the output current level becomes low (refer to FIG. 6, line c). Therefore, this type of circuitry is unable to produce a high level of output current, such as the one indicated by line a in FIG. 6, while providing a small variation in the output range and a low voltage/current conversion ratio, as expressed by the slope of the characteristic line.
Other examples of such v/c conversion circuits include a circuit disclosed in a Japanese Patent Application, First Publication, H1-170206, FIG. 1, which behaves in the same manner as the circuit shown in FIG. 5, and a high resistor value is needed to realize a v/c conversion circuit having a small conversion factor. Also, once the conversion factor (i.e. resistor value) is decided in a device, its output current is determined uniquely, and it is not possible to generate variable levels of output current. A circuit disclosed in FIG. 4 of the above-mentioned reference, is reported to provide a low v/c conversion factor without using a high value resistor, by relying on a biasing current I.sub.E and a resistor R.sub.E so as to make the v/c conversion factor low; however, this circuit can produce only very low levels of current.